Conventionally, in a cold region and the like, in order to prevent freezing of coolant water for cooling an internal combustion engine, an antifreeze liquid such as a long-life coolant (LLC) is mixed in the coolant water and used. Japanese Patent Laying-Open No. 2007-247506 (PTD1) discloses a method for detecting a concentration of an antifreeze liquid mixed in coolant water. According to this technology, antifreeze liquid concentration estimating means estimates a concentration of the antifreeze liquid based on a current value detected when a predetermined control is performed by pump controlling means in the case where a predetermined condition is satisfied.